1. Field of the Invention
The present invention relates to piezoelectric devices including a piezoelectric thin film and methods for manufacturing such piezoelectric devices.
2. Description of the Related Art
Piezoelectric devices including a piezoelectric thin film are currently being developed. An example of such a piezoelectric device is a surface acoustic wave device disclosed in Japanese Unexamined Patent Application Publication No. 6-326553.
FIG. 1A is an external perspective view of the surface acoustic wave device in Japanese Unexamined Patent Application Publication No. 6-326553. FIG. 1B is a sectional view taken along line A-A′ in FIG. 1A. The surface acoustic wave device shown in FIGS. 1A and 1B includes a non-piezoelectric substrate 150, an inorganic thin film 140 formed on the surface of the non-piezoelectric substrate 150, a piezoelectric thin plate 120 formed on the surface of the inorganic thin film 140, and comb-shaped electrodes 130 and 130′ formed on the surface of the piezoelectric thin plate 120. The inorganic thin film 140 is formed of, for example, a silicon oxide film. The piezoelectric thin plate 120 is formed of, for example, lithium niobate or lithium tantalate.
The piezoelectric thin plate 120, which is formed of lithium niobate or lithium tantalate, has pyroelectricity and high insulation properties. This causes a problem in that, when a piezoelectric device is manufactured using the piezoelectric thin plate 120 formed of such a material, a build-up of pyroelectric charge generated in the piezoelectric thin plate 120 damages the comb-shaped electrodes 130 and 130′ formed on the piezoelectric thin plate 120.
Accordingly, in the related art, a technique is used in which reduction treatment is performed on the piezoelectric thin plate 120 to decrease the insulation properties of the piezoelectric thin plate 120 so that a pyroelectric charge generated in the piezoelectric thin plate 120 flows from the piezoelectric thin plate 120 to the non-piezoelectric substrate 150.
However, in the piezoelectric device having the inorganic thin film 140 (oxide film), which is formed of a silicon oxide film, between the non-piezoelectric substrate 150 and the piezoelectric thin plate 120, which is extremely thin with a predetermined thickness (for example, 1 μm) or less, oxygen is supplied from the inorganic thin film 140 to the piezoelectric thin plate 120. For example, when the piezoelectric device is mounted on a module board, heat treatment is performed at 230° C. or higher. Even if the heat treatment is performed in a reflow oven in a reducing atmosphere, oxygen is supplied from the inorganic thin film 140 to the piezoelectric thin plate 120. This oxidizes a group of 5 to 100 atoms in the piezoelectric thin plate 120 that are adjacent to the oxide film and therefore removes the reduced state of the piezoelectric thin plate, thus forming an oxide layer 121 with high resistivity in the piezoelectric thin plate 120. As a result, as shown in FIG. 2, the oxide layer 121 prevents a pyroelectric charge generated in the piezoelectric thin plate 120 from flowing to the non-piezoelectric substrate 150, thus leaving a build-up of pyroelectric charge in the piezoelectric thin plate 120.
Thus, the piezoelectric device having the oxide film between the extremely thin piezoelectric thin plate 120 and the non-piezoelectric substrate 150 has a problem in that a build-up of pyroelectric charge in the piezoelectric thin plate 120 damages the comb-shaped electrodes 130 and 130′ formed on the piezoelectric thin plate 120. In particular, the oxidation of the piezoelectric by the oxide film has a greater impact on a device including an extremely thin piezoelectric than a conventional device including a piezoelectric substrate.